[This post is based on the SharePoint 2016 Preview — Functionality is not guaranteed to be supported, configured identically, or exist in RTM]
SharePoint 2016 includes a new response header. Named X-SP-SERVERSTATE, this one is very simple. It just tells you if the site is Read Only or not.
The header is returned on calls to the API endpoint, such as “/_api/search/searchcenterurl”, and calls to “/_vti_bin/”.
The value if the site is marked as SPSite.ReadOnly = true is X-SP-SERVERSTATE: ReadOnly=1. Otherwise, it will read X-SP-SERVERSTATE: ReadOnly=0.